1. Field of the Invention
This invention relates generally to the field of color characterization of color input devices, and specifically relates to generating a color characterization model for performing transformation from a device-dependent color space to a device-independent color space. The color characterization model is based on measured color data points.
2. Description of the Related Art
Traditional color management systems (CMSs) use color characterizations of color input devices to derive color transformations that transform color coordinates between a device-dependent color space that depends on the device, and a device-independent color space. In particular, color characterization for a color input device such as a camera or scanner can be derived by first capturing a target consisting of predesignated color patches. In a device-dependent color space such as the RGB color space, this results in an RGB bitmap image in which the color of each patch is encoded in an RGB value.
Given a particular target, the color transformation between the device-dependent and device-independent color spaces can be modeled reasonably by polynomials of low degrees. The mathematical method or technique for fitting these polynomials is known as polynomial regression.
A problem arises in the use of polynomial regression based on measured data points. In particular, the use of polynomials tends to overshoot or undershoot beyond the range of the measured data points in a given space. An aspect of this problem is that the device-independent color space typically has constraints associated with it that are imposed by the physical range of the quantities being modeled. In other words, a color in the device-independent color space is typically represented by three coordinates. These three coordinates must satisfy certain constraints and relationships among them to represent a physically possible color. To impose these constraints, domain knowledge rather than the use of purely statistical techniques is required.
If the measured data points from the color input device uniformly fill in the device-dependent color space, the problem of overshooting or undershooting beyond the range of the data points does not arise. However, data points measured from a standard target are rarely distributed uniformly in the device-dependent color space. For instance, measured RGB points typically fill up the inner portions of the RGB cube, or reside in a lower dimensional manifold of the RGB cube. When this happens, the regressed polynomials based on the measured data points are inaccurate in predicting device-independent values beyond the range of the measured data points.